Apaf-1 (apoptotic protease activating factor 1) is a cytosolic protein involved in cell death or apoptosis. When Cytochrome c is released from the mitochondria, it interacts with Apaf-1 and dATP to form the apoptosome, a large oligomeric protein complex which can activate caspase 9.

The crystal structure of this protein was solved in April of 2005 by the laboratory of Yigong Shi[1].

The actual mechanism for this reaction is still debated though work published in April of 2006 from the laboratory of Guy Salvesen suggests that the apoptosome may induce caspase 9 dimerization and subsequent autocatalyzation [2].